Though Rheumatoid arthritis affects the joints, it’s a disease of the immune system, which normally protects us from infection by attacking viruses and bacteria. For reasons no one fully understands, RA causes the immune system to go awry and mistakenly attack healthy cells such as the synovium, a thin membrane that lines the joints. As a result of the attack, fluid builds up in the joints, causing pain and inflammation. Over time this can wear away the cartilage and erode bone, causing a lack of function and mobility. In most people, the inflammation usually becomes systemic, affecting organs such as the skin, heart and lungs.